(a) Field of the Invention
The present invention related to a method and an apparatus for modulating a baseband signal in a beam space MIMO.
(b) Description of the Related Art
Recently, a MIMO technique has been adopted in a variety of communication techniques. The MIMO technique has a merit of increasing a data rate and maximizing frequency efficiency. The MIMO technique has been adapted in a WiBro system, a 3GPP cellular communication system, as well as an IEEE 802.16 and IEEE 802.20 of a portable Internet system.
Transmission performance in the MIMO communication system is generally increased proportional to a number of antennas. Accordingly, the number of antennas must be increased to maximize the MIMO performance, whereby a number of RF (radio frequency) chains is also increased. When the number of antennas is increased, realization complexity thereof is increased and a size of the system is increased such that there is a drawback that the number of antennas may not be increased much. In order to escape this constraint, research to obtain the MIMO performance by using one RF or a small number of RF chains has recently progressed. As a representative example, there is a beam space MIMO technique using an ESPAR (Electrical Steering Parasitic Array Radiator) antenna or a load modulation antenna.
The beam space MIMO technique has a different point from a general MIMO technique in the antenna/RF aspect and a baseband aspect.
First, the antenna/RF aspect will be described as follows. The general MIMO technique configures the antenna by using a plurality of active antenna elements, however the beam space MIMO technique configures the antenna by using one or a small number of active antenna elements and a plurality of parasitic antenna elements. In a merit of this beam space MIMO technique, a broadening effect of the antenna number may be obtained through a plurality of parasitic antenna elements and a distance between the plurality of parasitic antenna elements may be reduced. Also, since the beam space MIO technique uses one or the small number of RF chains, the RF portion is not complicated and may be realized with a small size.
The baseband aspect will be described as follows. The general MIMO technique uses the plurality of active antenna elements and radiates the modulated baseband signal for each active antenna. Accordingly, by a phase difference and a size difference due a path through which each signal is transmitted, the phase and the magnitude of the signal radiated through the active antenna are finally determined. Each signal is passed through each path, and the phase and the magnitude of each signal radiated in the antenna is affected by a phase noise of a local oscillator or a RF impairment such as an IQ imbalance of the transmission path. The RF impairment is measured to be compensated in the baseband aspect, and the system may be configured by considering this space when configuring the RF. However, since the RF impairment (i.e., IQ imbalance or phase noise) are sharply changed and has almost a constant value, there is no problem under the compensation of a longer period.
In the beam space MIMO technique, if an entire antenna beam pattern generated by one or the small number of active antennas and the plurality of parasitic antennas is exploded, a plurality of orthogonal beams are generated. The baseband signal is mapped to the plurality of orthogonal beams. By an impedance value of the baseband signal and the plurality of parasitic antennas, a current value flowing to the plurality of parasitic antennas is changed, thereby the radiation signal mapped to the beam is finally generated. Also, the phase and the magnitude of the signal that is finally radiated is determined by the current value. That is, the phase and the magnitude of the signal that is finally radiated is changed by changing a load value (the impedance value of the plurality of parasitic antennas), thereby generating the desired radiation signal.
Meanwhile, the plurality of parasitic antennas are realized by a variable impedance element, and the value of the variable impedance element is affected by the signal that is generated in the baseband. That is, the load value must be changed whenever the value of the signal generated in the baseband is changed. Accordingly, tuning is required whenever the baseband signal is changed, and it is necessary to tune the beam space MIMO technique with a faster cycle than the general MIMO technique.
The current required by each antenna element (i.e., a plurality of parasitic antenna) is calculated with a ratio type by corresponding to a ratio of the signal generated in the baseband, and the value of the impedance of each antenna element is determined by using the current ratio. However, this conventional method does not matter in a single carrier system, however it causes the following issues in a wideband system or a multi-carrier system. When the variable impedance is realized as a capacitor component or an inductor component, since a reactance component of the variable impedance is changed along the frequency, the issue may be generated in the wideband system or the multicarrier system.
In other words, it is necessary to consider the change of the value of the variable impedance due the frequency in the beam space MIMO. The signal may not be normally transmitted by the change of the value of the variable impedance due to the frequency. When outputting the same symbol with different frequency, the phase difference and the magnitude difference are not constantly radiated. Accordingly, the phase and the magnitude of the signal that is finally radiated do not preserve the phase and the magnitude of the baseband. When measuring the phase and the magnitude of the channel for a reference signal distributed to the frequency axis, problems may occur in reliability.
Meanwhile, in the beam space MIMO, since there is only one RF chain, different from the conventional, the several signals may be simultaneously received. Accordingly, by appropriately controlling the load value in the baseband through the beam space MIMO receiver, a beam basis pattern is rotated in all directions along a time within one symbol duration to obtain the information for the MIMO signal while scanning each Beam Basis pattern. An ADC sampling frequency is changed depending a number of the Beam Basis patterns, and the ADC sampling frequency depends on the number of the Beam Basis patterns. In this case, a length of one symbol duration is constant. Accordingly, the beam space MIMO receiver changes the ADC sampling frequency to be operated with a sample number of each Beam Basis pattern maintained constantly. That is, if the number of the Beam Basis patterns scanned in one symbol duration is increased, the ADC sampling frequency is increased and a time interval of one Beam Basis pattern in the symbol duration is shortened, thereby an S(I)NR is decreased. Accordingly, when receiving a lot of streams, the S(I)NR is structurally reduced and a very small portion of streams may be received simultaneously. Thus, the merit for the MIMO mainly disappears. Accordingly, a receiving method that minimizes the performance reduction is required.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.